This invention relates generally to wafer slicing and more particularly to quality control in wafer slicing.
Internal diameter saws having an annular saw blade with a diamond-coated cutting edge on its inner diameter are conventionally used to slice thin wafers from an ingot of monocrystalline semiconductor material. The wafers are sliced by either advancing the ingot against the blade or advancing the blade against the ingot. The axis of rotation of the blade may be either horizontal or vertical.
Cutting resistance forces during slicing cause the blade to deflect transversely to the cutting direction and out of the plane of the blade. If sufficiently large, this deflection can produce undesired warpage, saw marks and thickness variations in the wafers sliced. The warpage cannot be eliminated by operations subsequent to slicing such as lapping. Even though saw marks and thickness variation can be eliminated by increasing stock removal in processing operations after slicing, the resultant material loss and time inefficiencies make this solution highly undesirable.
The cutting resistance encountered by the blade is primarily a function of the condition of the diamond-coated cutting edge of the blade. In order to limit the cutting resistance forces and concomitant blade deflection, it is standard industry practice to dress the cutting edge of the blade using special dressing stones. The dressing stone is pushed in a specific direction and with a particular pressure against the cutting edge of the blade according to the measured warpage of wafers sliced by the saw. The dressing operation is critical to controlling the quality of the wafers sliced, but in practice is highly dependent upon the subjective judgment of the saw operator and does not always result in the desired correction on the cut following the dressing operation.
Various equipment and methods have been employed for controlling the quality of sliced wafers. European patent application No. 88101301.5 describes an apparatus and method for slicing a wafer in which, after a wafer is sliced, the end face of the ingot is ground. The disclosed apparatus and method reduce warpage, but do not reduce saw marks or thickness variation because the apparatus and method do not even attempt to reduce blade deflection. European patent application No. 91107503.4 describes a method in which the annular blade of an internal diameter saw along its cutting edge is intentionally deflected prior to slicing through application of a force perpendicular to the blade surface near the cutting edge. More specifically, the application discloses air jet nozzles located with respect to the annular saw blade for directing a jet of air against the blade surface at positions where the blade enters the ingot, and where it exits the ingot, respectively. Blade position sensors are located for detecting blade position at the same two locations. A control unit is capable of adjusting the pressure applied to the blade surface through each nozzle in response to the detected blade deflection. This system is able to minimize blade deflection at the locations where the air jets impinge the blade, but does not prevent blade deflection within the ingot of sufficient magnitude to reduce warpage, thickness variations and saw marks.